Neuropsychologically relevant mental abilities are studied in healthy men at different ages, in patients with clinically-diagnosed Alzheimer's disease, and in adults with Down syndrome at different ages. Tests are administered to evaluate intelligence, memory, language, visual attention, visuoperceptive and visuoconstructive ability, and perceptual-motor speed. Age-related differences in general intelligence and visual memory in our sample of healthy men, ranging in age from 20 to 83 years, were found to be smaller than the differences reported in normative studies of non-health-screened adults. Visual memory and the discrepancy between verbal and visuospatial ability were not correlated with regional cerebral metabolic rates for glucose (rCMRglc) as measured by positron emission tomography (PET) and 18-Fluorodeoxyglucose. Patients were divided into mildly, moderately and severely demented groups, based on the Mini-Mental State Examination. The discrepancy between verbal and visuospatial abilities was correlated with lateral asymmetry of cortical rCMRglc in patients with moderate Alzheimer's disease, but not in patients with mild Alzheimer's disease. Asymmetry of visual attention to the right and left sides of extrapersonal space was also related to lateral cerebral metabolic asymmetry in moderate Alzheimer's disease. Older Down syndrome adults perform worse on mental abilities tests than did younger subjects. Immediate verbal memory appears to be less affected by age in Down syndrome than are other abilities.